HOUSTON, AUG. 20 -- President Bush tonight portrayed his Democratic opponent as a risk to the nation's future and made the case to cheering Republican partisans that he is the man to reap the rewards of democracy's triumphs around the globe and use them to secure prosperity at home.

The president offered few new prescriptions for the ailing economy but presented, instead, a broad "agenda" that he said showed he was for lower taxes, less government and less government spending while his rival Bill Clinton is for more of all of them. He said he would offer at the start of a second term across-the-board tax cuts, spending reductions and a plan to reduce the debt by allowing taxpayers to earmark up to 10 percent of their taxes for debt reduction.

Bush sketched a vision for his next four years that linked his foreign policy successes to America's needs at home, played off his wartime service and experience in wrestling with international crisis to raise doubts about Clinton's readiness for the Oval Office and posed as the major question of the fall campaign: "Who do you trust to make change work for you?"

Addressing the greatest broken promise of his first term, his 1988 "read my lips" pledge not to raise taxes, the president offered a mea culpa and his version of what happened. "When it comes to taxes, I've learned the hard way," he said, calling the 1990 budget agreement to raise taxes and limit spending a case of "bad judgment."

"With my back against the wall," Bush said, he agreed to a "Democratic" tax increase that turned out to be a mistake. At the time, Bush volunteered to put "revenue enhancers" into a package of spending cuts and new taxes. During the primaries last spring, when challenger Patrick J. Buchanan was lambasting Bush for breaking his pledge, the president abandoned his support for that budget deal and called it a mistake because it was bad politics.

Tonight Bush asked: "Who do you trust in this election? The candidate who raised taxes one time and regrets it, or the other candidate who raised taxes and fees 128 times, {a reference to Clinton's tenure as governor of Arkansas that Clinton disputes} and enjoyed it every time?"

Bush's 56-minute speech, longer than Clinton's acceptance speech last month that was criticized for its duration, closed the four-day Republican National Convention and was described by his aides beforehand as one of the most important moments of his presidency. Trailing Clinton badly in the polls and facing a credibility crisis over his broken tax pledge and an electorate overwhelmingly unhappy with the direction in which the country has been going the past two years, Bush hoped to convinced people he has the better answer to the country's problems and conviction to follow through.

Bush was preceded on the podium by Vice President Quayle, who hammered home his theme that Republican values are at one with those of the nation, and by a string of Republican officeholders, current and past, who harshly attacked the Democratic Congress, lauded Bush's foreign policy leadership and his tenure.

Bush began with a description of how the world has changed since he became president, ticking off the fall of the Berlin Wall, the peace talks between Arabs and Israelis, the freeing of American hostages in the Mideast, the end of the Soviet Union, and then making a sarcastic reference to Clinton. "If I had stood before you four years ago and described this as the world we would help to build, you would have said, 'George Bush, you must be smoking something. And you must have inhaled.' "

Linking those changes to a nation now free of the fears of nuclear war and freer to pursue prosperity at home, Bush defended his concentration on foreign policy in much the same way Secretary of State James A. Baker III did last week when he announced he would be resigning to become White House chief of staff. "I seized those opportunities for our kids and our grandkids, and I make no apologies for that," he said to the roar of the delegates.

And referring to the war in the Persian Gulf, Bush spoke of the risk of remaining "wolves in the woods" and ridiculed Clinton's statements when Congress was considering Bush's call to war in January 1991.

"What about the leader of the Arkansas National Guard, the man who hopes to be commander-in-chief? Well, I bit the bullet, and he bit his nails," Bush said. Clinton's policy, he said, "can be summed up by a road sign he's probably seen on his bus tour, 'Slippery When Wet.' "

Bush said the "defining challenge of the '90s is to win the economic competition -- to win the peace" and said the Republican vision for doing that is open markets for American products, lower government spending, tax relief, help for small business, legal, educational and health reform, and job training problems.

On the economic front, he said he would propose tax cuts but did not specify for whom or how much, and proposed one new idea -- allowing taxpayers to specify on their tax returns that up to 10 percent of their taxes go for debt reduction alone. To make up the lost revenue, he would require that for every tax dollar set aside to cut the debt, ceilings on federal spending would be lowered, forcing less government spending on programs.

That proposal is a favorite of some House Republicans who passed it on to the White House. Rep. Robert S. Walker (R-Pa.) has proposed such as plan, arguing that if every taxpayer checked off the full 10 percent, the budget could be reduced by 4 percent.

A senior official here said Bush had decided against specific economic proposals because Congress would not take them up anyway this year and argued than a convention acceptance speech is the place for a broad vision, not a laundry list of detailed plans.

The president did tick off the tax proposals he has sought, and failed to get from Congress the past four years, from the reduction in capital gains taxes to an increase in the personal exemption. And he heaped scorn on Clinton as a Democrat who reaches to tax everything from individuals to small business to large business, risking further loss of jobs and prosperity.

As virtually every speaker has done the last three days, Bush also made the case for a Republican Congress to break the "gridlock" he says keeps him from getting his programs approved. "I know Americans are tired of the blame game. . . . " Bush said, "I don't like it either. Neither should you. But the truth is the truth: Our policies haven't failed, they haven't been tried."

He warned that if voters put a Democrat in the White House to match the Democratic Congress, they would produce a "dangerous combination": a rubber-check Congress and a rubber-stamp president.

In calling for a Republican Congress for the first time in almost five decades, Bush made the case that this is the year, because of redistricting and retirements, it can be done. And he pledged that with "150 new members" he will begin anew in his second term because each will have a "fresh view" of the nation's problem. He pledged to meet with every new member to "lay out my case" before a new session opens in January.

In a conclusion that echoed the optimistic themes of his predecessor, Ronald Reagan, Bush spoke of the nation's uneasiness and anxiety about the future, but said, "From where I stand, I see not America's sunset, but a sunrise." And he made his appeal, "I ask you to consider, now that the entire world is moving our way, why would we want to go back their way?"

Bush was preceded to the podium by Quayle, whose aides had described his appearance as a critical opportunity to reverse the negative image implanted in the public mind four years ago when Bush selected him.

Aides to Quayle had promised a more personalized account of his formative years, and a biographical film and a few sentences in his speech were attempts to portray the vice president not as a wealthy child of a privilege but as a product of middle-class, small-town America.

In contrast to his wooden performance four years ago in New Orleans, Quayle appeared animated and clearly enjoyed the enthusiastic response he got from the crowd. He used his moments before the huge convention crowd and television audience to return to his primary theme of the campaign: that a "cultural divide" separates Republicans from Democrats and that Republicans represent the right values.

"The gap between us and our opponents is a cultural divide," he said, "It is not just a difference between conservative and liberal; it is a difference between fighting for what is right and refusing to see what is wrong."

Quayle said Americans try to raise children to understand right and wrong "only to see their values belittled and their beliefs mocked by those who look down on America."

In a segment about an undefined "they" that was reminiscent of a rhetorical device often used by former president Richard M. Nixon, Quayle told his audience that his critics wished he had not gotten the nomination. "They don't like our values," he said, "They look down on our beliefs. They are afraid of our ideas," and "they will stop at nothing to destroy me."

But, said Quayle, they "have failed. I stand before you, and before the American people -- unbowed, unbroken, and ready to keep fighting."

Quayle attempted to tie the Democrats to a string of special interests in explaining why Clinton cannot be for change. "Bill Clinton talks about change but he can't really change America because the special interests won't let him."

He also mocked the portrayal of the Democrat ticket as moderates, trying to turn one of his own gaffes -- misspelling the word potato -- into a plus. "Well, if they're moderates, I'm a world champion speller," he said.

With all his closest aides -- and Bush himself -- acknowledging that Bush campaigns best as an underdog, that appealing image of an embattled fighter was played up by speaker after speaker, capped by Senate Minority Leader Robert J. Dole (Kan.), who engaged in a losing battle with Bush four years ago for the GOP nomination.

"Get ready to meet the real comeback kid," Dole told the cheering delegates, a take-off on Clinton's reference to himself in the early months of the Democratic primary race. "Four years ago, they said he couldn't win the nomination. You're looking at someone who knows better."

As former president Reagan had opened the convention Monday, former president Gerald R. Ford appeared this final night to hit a key theme of the four days -- the changes that have swept the world, from the fall of the Berlin Wall to the break-up of the Soviet Union were no accident.

"It was George Bush at the helm of our ship of state when freedom finally triumphed," Ford said. Dole earlier made a similar point, as did many other speakers, noting the military strength of the Reagan and Bush years and their tough stance helped destroy the United States' major enemy and such a result was no accident.